AN2281 datasheet(8/38 Pages) STMICROELECTRONICS

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Implementation on the ST7LITE35 microcontroller

AN2281

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3.1.2

Three-phase sinusoidal waveform generation

In order to produce the three-phase sinusoidal voltages, three of the four available PWM

outputs have been enabled. Since these three PWM signals must have the same frequency,

single timer mode has been selected (ENTNCR2=0).

To allow the necessary duty cycle updating, the OVFIE bit of the ATCSR register has been

set. This way an interrupt is generated every 1/fPWM seconds.

Furthermore, to reduce the acoustical noise introduced by the switching of the three-phase

inverter, a PWM frequency out of the audible range has been chosen. In particular, to

achieve the selected 15.625 KHz switching frequency, fCOUNTER has been fixed equal to

fCPU (8 MHz) and, consequently, using formula (3.1), the ATR1 registers have been written

with value 3584.

To reduce the contribution of the OVF interrupt service routine to the overall CPU load, the

calculations necessary for computing the three sinusoidal varying duty cycles are carried

out once per two PWM periods (that is once every two interrupts). This way, the number of

traced points per sine wave period (NP) is given by:

Since at least 18 samples per sine wave cycle must be traced to generate a sine wave with

a Total Harmonic Distortion minor than 5%, (3.2) limits the maximum sine wave frequency to

434 Hz that is equivalent to about 26,000 rpm for an one poles pair motor.

3.1.3

Third harmonic modulation

Basically, to provide the voltage needed for the PMAC motor, the reference PWM signal

could be a pure sine wave, but this kind of modulation has the drawback that it makes poor

usage of the DC bus voltage.

Adding a third harmonic modulation to the reference sine wave, allows the phase-to-phase

voltage amplitude to be increased without deteriorating current and phase-to-phase voltage

THD (a 120-degree phase-shift on the fundamental corresponds to a 360-degree shift for

the third harmonic). On this subject, the literature demonstrates that, if the third harmonic

amplitude is equal to one sixth of the fundamental one, it is possible to increase the phase-

to-phase voltage amplitude by 15% with respect to the pure sine wave approach.

(3.2)

2 * fSINE

fPWM

NP =

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